Our experiments in the field of transplantation immunology will continue with special emphasis on studies of the mechanism of "graft adaptation" in which changes occurring in both the graft and the host will be evaluated under selected circumstances. Two experimental systems will be highlighted. The first will involve primarily vascularized organ transplants in mice, especially transplants of the kidney and heart by microsurgical techniques. The other will consist of a series of experiments in which rat skin xenografts will be transferred to immunosuppressed mice and their susceptibility tested to the toxic effects of passively infused mouse anti-rat antibody at various times. In both systems we have evidence that "graft adaptation" occurs.